I'm in the middle of a garage addition with finished 2nd floor. By code I need to have 2 sheets of 5/8 drywall on the ceiling and wall between the garage and house. Wondering if I should liquid nail the sheets together and install them as if it was one sheet (with a lift)? If I install it one sheet at a time I could stagger the seams on the 2nd layer, but it'll be twice the work. Its a 3-car garage, 70 sheets on the 9 ft. ceiling. I have a lot of lights, boxes, etc. to cut around.

Also, anyone know what type of R factor the 1.25" of drywall will add to the ceiling? I'm insulating now and just curious.

I assume you're making a double thick wall for fire rating purposes. If that's the case, you don't want to glue them up together...the seams where two pieces meet should be staggered. Thick brick wall with staggering joints.

If it's not for fire rating purposes, then you can glue them up, although you might have a tough time smoothing out the joints.

I would not recommend gluing - the whole idea is fireproofing - liquid nails or other adhesives are flammable and defeat the purpose - if a fire burns through the first layer and hits the adhesive it would just fuel the fire and add more toxic gases - if the garage is attached to your home, yu don't want to take this chance. Besides, those suckers are heavy enough! Combined it would be a load and a half!